Method of making extended surface heat exchangers



Feb. 5, 1952 D. DALlN 2,584,189

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BMW .Uavzd 17512222 UNITED STATES PATENT OFFICE METHOD OF MAKING EXTENDED SURFACE HEAT EXCHANGERS David Dalin, Stenkullen, Ronninge, Sweden, assignor to A/B Svenska Maskinverken, Sodertalje, Sweden, a corporation of Sweden Application March 21, 1949, Serial No. 82,572 In Sweden October 16, 1948 12 Claims.

My invention relates generally to the fabrication of an article or piece of apparatus having a metal base wall and closely spaced wire or rodlike metal elements projecting therefrom and element and the base wall and substantially uniform distribution of heat throughout the base wall are assured.

The use of electric welding to achieve the purrefers more particularly to a method of making 5 poses of this invention suggests itself, and others extended-surface heat exchangers. confronted with a somewhat similar problem The oopending application, Serial No. 53,004, have attempted to find the answer therein. The filed October 6, 1948, now Patent No. 2,469,635, United States patent to Joseph H. Cooper, No. in which I am a co-inventor, disclose that the 2,337,294, issued December 21, 1943, is an exmost eifective extended surface for heat example. Cooper used the so-called electro-perchangers consists of wire or rod-like elements cussive method of welding, but that method is of copper or other metal having a high coefficient not suitable for the attachment of extended surof thermal conductivity extending out from the face elements to the base wall of a heat extube or other base wall and disposed in parallel changer. The high voltages it entails are dangerclosely spaced relationship. The need for intious. More important, however, is the fact that mately and securely fastening the extended sur- With electro-percussive welding the element is so face elements is, of course, obvious, as is also the secured to the base wall that at the most the desirability of good thermal conductivity at the area of the junction is limited to the cross secjunction. tional area of the element, and consequently The attainment of these obvious objectives, 2 adequate heat distribution over the whole area however, posed a baffling problem aggravated by of the base wall is not achieved. the fact that any means employed to fasten the Arc welding is entirely unsuitable for the fabrielements had to lend itself to production methods cation of extended surface heat exchange deof fabrication for otherwise the cost factor would vices, since be a serious deterrent to the adoption of this ideal 1. Its extremely high temperatures would heat exchanger structure. weaken and distort the base wall; and

Another important consideration having a 2. The time involved in making each weld and bearing upon the manner in which a heat exthe space required therefor would render the changer of this type might be fabricated is the method economically impracticable. need for even distribution of heat throughout the Resistance welding, and more particularly the entire area of the base wall. If the influence of branch thereof known as projection welding, the extended surface upon the base wall, i. e. the therefore presents the only possibility. It ha the abstraction of heat therefrom or the feeding of advantage of low voltage, the voltage at the weldheat thereto by the extended surface elements, ing point seldom exceeding three volts. But the reaches only a small area of the base wall diauthorities have generally agreed that resistance rectly adjacent to the point of attachment of welding is not satisfactory for the welding of each element, the resulting concentrations of copper and other metals having a high coeffiheat will cause serious damage to the base wall. cient of thermal conductivity. For instance, in the case of a boiler tube con- Note for instance the following: taming Water and having h gases over 40 1. Resistance Welding Manual 1946. the exterior thereof, the high concentrations of I heat at localized areas brought about by uneven Page J d g is not generally heat distribution in the tube wall causes dry recommended for pure copper spots on the water side of the wall. This is com- Page 107.-Projection welding: Copper and monly known as the Leidenforst phenomenon. the red brasses are considered unweldable by Such dry spots result in overheating and weakenthis method. mg of the tube 2. Mallory Resistance Welding Data Book 194%. Hence, any means employed to attach the ex- Page 126 SpOt Welding data. tended surface to the base wall should take into Pure+c steel (mild steel) Weldabmty account the need for uniform heat distribution Very p001 LOW Weld strength. No actual throughout the j f of the base Weld nugget occurs, a stick is obtained.

In full recognition of these requirements the present invention has as its purpose to provide Weldmg f 1942- a method of attaching wire or rod-like extended Page 37a-Re51swnc? spot and 5 Weldmg surface elements of copper or other highly concopper h baen med tunes but not with sufficient success to consider it commerductive metal to a base wall whereby a perfect bond is secured between the extended surface elements and the base wall, and whereby optimum conditions of heat transfer between each cially practical.

Page 872.--Resistance spot and seam Welding of deoxidized copper: When fabrication by this method is desired, it is much more to the point to use a silicon brass or copper-nickel alloy,

I have found, however, that contrary to the generally accepted opinion of the welding art, resistance welding can be adapted to the instant problem and I have found that with my novel application of resistance welding it is possible to not only securely bond the extended surface elements to the base wall with a rapidity which fully satisfies the requirements of production methods of fabrication, but also to so attach the extended surface elements that the area of the junction between each element and the base wall is considerably larger than the cross sectional area of the element per se. Asa result the heat transfer between the elements andthe base wall is suinciently extensive to assure substantially uniform heat distribution throughout the base wall.

In explanation of this last statement it may be briefly noted that with my method a foot or enlarged base is formed on each element as it is welded to the base wall, which foot covers an area of the base wall substantially greater than could be covered by the cross section of the element alone. It is this feature, together with the close spacing of the elements over the base wall, which gives the resulting structure its good heat transfer characteristics. It also assures the desired mechanical strength in the junction since though the junction may not be as strong per unit of area as the metal of the element per se, its increased area more than makes up this difierence.

With the above and other objectives in view which will appear as the description proceeds this invention resides in the novel method and application thereof substantially as hereinafter more particularly described and defined by the appended claims, it being understood that such changes in the precise embodiment of the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate different types of extended surface heat exchangers which may be made by the application of the method of this invention and illustrate diagrammatically one specific manner of practicing the same.

In these drawings:

Figure 1 is a cross sectional view through a heat exchanger especially adapted for use in steam boilers since the extended surface is mounted on the outside of a tube through which boiler fluid is adapted to flow;

Figure 2 is a view in elevation of a portion of the heat exchanger shown in Figure 1;

Figure 3 is a cross sectional view through a heat exchanger adapt-ed for installations wherein the fluid medium at one side of the dividing wall has substantially the same alpha value as the fluid medium at the other side thereof so that extended surface should be used on both sides of the wall;

Figure 4 is a detail cross sectional view on an enlarged scale through Figure 3 on the plane of the line 4-4;

Figure 5 is a front elevational view of a segment of a brake drum having extended surface elements attached thereto in accordance with this invention;

Figure 6 is a cross sectional view through Figure 5 on the plane of the line E-B;

Figures 7 and 8 are detail sectional views, on a greatly enlarged scale, through the junction between one extended surface element and the base wall, to illustrate the difference between a weld of the character contemplated in the pat ent to Cooper No. 2,337,294 and a weld formed in accordance with this invention;

Figures 9 and 10 are plan views of sections of a heat exchanger, respectively illustrating the areas or thermal influence of the junctions in Figures 7 and 8;

Figure 11 is a diagrammatic view illustrating the first stepin the method of his invention in the application of extended surface elements to a tube;

Figures 12 and 13 diagrammatically illustrate intermediate and final steps in the attachment of the elements to a tube;

Figure 14 diagrammatically shows the meth- 0d of attaching the elements to both oi the base wall to produce the structure shown in Figures 3 and 4;

Figure 15 is a graph which may be used to determine the amperage of the welding current for the welding of copper elements a steel base wall, providing the cross sectional area of the element is known;

Figure 16 is a graph showing the correct welding force or pressure for the same combination of metals;

Figure 17 is a graph by which the projecting length of the element may be determined; and

Figure 18 is a graph showing the proper welding time for the welding of copper elements to a steel base wall.

Referring particularly to the accompanying drawings and considering first the mechanical structure of the heat exchangers produce" by the practice of this invention, the numeral designates the base wall in each of the several heat exchangers shown and to which in each in-- stance the extended surface elements are attached in closely spaced relation. In the heat exchanger shown in Figures 1 and 2 the base wall 5 is a steel tube especially adapted for use in high pressure boilers, and the extended surface elements 6 are of copper or some other metal having high thermal conductivity and their points of attachment to the tube are uniformly spaced around the circumference of the tube.

In the heat exchanger of Figures 3 and 4 the base wall is a flat dividing sheet between two passages or ducts through which two fluids, as for instance combustion gases and may now. Since in this case both fluids have the alpha value the extended surface elements 6 project from both sides of the wall.

In the case of the brake drum shown in Figures 5 and 6, the elements 6 project from only one side of the base wall 5 to dissipate the heat generated on the inner face of the wall during application of the brake.

As taught in the aforesaid copending application theextended surface elements 8 are rodlilze and of relatively small diameter and are arranged in parallel closely spaced relationship.

In the case of the brake drum the elements are parallel in one direction while they not strictly parallel in the other direction but because of the relatively large diameter the drum their radial disposition approaches parallelism. In the case of the boiler tube shown in Figures 1 and 2 the individual elements 6 are bent as at I to dispose the same in parallel relationship.

Though the elements 6 are preferably round in cross section they may be of any desired cross sectional shape.

Where the extended surface elements 6 project from each side of the base wall as in Figures 3 and 4, the elements on one side of the wall are preferably axially aligned with those on the other side thereof.

As best shown in Figure 7 the junction between each element and the supporting wall covers an area substantially larger than the cross sectional area of the element per se. In other words, each extended surface element 6 has an enlarged foot or base 8 spread out over and intimately joined to the base wall 5. By virtue of this enlargement of the attached ends of the elements it and the close spacing of the elements the thermal influence of the elements reaches all portions of the base wall with almost equal elfect. This is, of course, of tremendous importance in that type of heat exchanger in which the base wall is of a metal having a lower coefficient of heat transfer than the extended surface elements since it assures uniform heat distribution throughout the base wall and precludes high con centrations of heat at localized areas.

A comparison of Figures 7 and 8 and 9 and 10 graphically illustrates the effectiveness of the expanded foot 8. In Figures 7 and 8 the dotted lines 9 represent thermal conductance paths and the lines It, which are drawn at right angles to the lines 9 within the wall 5, are isotherms denoting the same temperature drop in both figures. Thus, for instance if the distance between adjacent isotherms I represents a five degree temperature drop, it will be seen that the temperature drop between the bottom of the foot 8 (Figure 7) and the opposite side of the base wall is considerably less than ten degrees whereas for the same distance in the structure of Figure 8 the temperature drop is twenty-five degrees. Also, because of the expanded foot 8 the iso therms in reach out much farther as shown by a comparison of Figures 9 and 10.

From practical investigations and actual measurements I have determined that when the elements are spaced to achieve optimum heat exchange conditions as set forth in the aforesaid copending application, the area of the foot, at its junction with the base wall, should not be less than one and three-fourths times the cross sectional area of the element per se.

Attention is directed to the fact that the junction denoted by the line I I, between the foot 8 and the base wall extends down into the base wall a distance, so that in the case of the structure shown in Figures 3 and 4, the metal of the elements is brought close together. Though to the naked eye this junction between element and base wall appears as a sharply defined line, it is most likely a zone in which the molecules of the element metal intermingle with those of the metal of the base wall. In any event, when my method is properly followed the joint is sound and fully as strong as the element per se.

The practice of the method by which the e1ements are attached to only one side of the base wall, as in the structure of Figure 1, is diagrammatically illustrated in Figures 11, 12 and 13, and in Figure 14, when the elements are fixed to both sides of the base wall as in Figure 3. In general it involves the technique of so-called resistance welding which is characterized by the fact that the current used is of relatively high amperage and low voltage. As already noted the voltage I employ is between one-half (0.50) and three (3) volts at the welding point, which means that the open circuit voltage would be between approximately one (1) and thirty volts, and

iii)

the amperage is between 5,000 and 150,000 amperes depending upon the metals used and the cross sectional area of the elements as will be more fully explained hereinafter. The heat for the formation of the Weld results from the high electrical resistance at the junction of the two pieces.

Resistance welding which includes projection welding the branch thereof most closely related to the method of this invention, is, of course, relatively old, but heretofore the authorities have always considered it impossible by this method to directly weld a piece of small cross section and high thermal conductivity endwise to a large surface, and especially to a large surface of dissimilar metal. This supposition has been especially well entrenched in the case of the endwise attachment of copper wire or rod-like elements to a sheet surface. But the present invention accomplishes this heretofore seemingly impossible result in a practical manner fully adapted to production methods of fabrication.

The method consists in grasping the rod-like element in a chuck l2 at a specified distance back from the end of the element to be fixed to the base wall, and pressing the element against the base wall. Where the elements are secured to only one side of the base wall, it is supported on a saddle I3 whcih serves as an electrode since it provides the electrical connection to the base wall. Where the base wall is a tube (as in Figure 1) the saddle is arcuate. For a flat plate it would be fiat, and where the elements are to pro ject from both sides of the base wall as in Figure 3, two chucks I2 are used and the wall 5 is supported in any suitable manner between them.

It is, of course, to be understood that the surfaces of the base wall and the ends of the elements which are to be fixed to the base wall must be thoroughly clean before the weld is made.

After the parts have been pressed together,

and while the pressure is maintained an electric current of high amperage and low voltage is passed through them and across the junction therebetween, this being done (in Figures 11, 12 and 13) by connecting each half of the chuck with one side of a power source l4 through leads I5 and connecting the saddle I3 with the other side of the power source through a lead [6, and in Figure 14, by connecting the two halves of one chuck l2 with one side of the power source and connecting the two halves of the other chuck with the other side of the power source.

To preclude objectionable heating of the chuck and the adhesion of the element 6 thereto, the chuck is cooled in any suitable manner as by circulating a coolant through a jacket [1.

By virtue of this cooling the portion of the element gripped and that part thereof extending above and beyond the chuck is kept below annealing temperatures to assure that the desired hardness of the element will not be disturbed during the welding operation. The portion of the element directly adjacent to the supporting surface, however, is annealed. This is desirable since in the formation of the heat exchanger shown in Figures 3. and 2, for instance, all of the elements when applied are radial to the tube as shown in dotted lines in Figure 1 and are later bent down into parallelism.

During the first brief instant that the current is on the lower end of the element becomes soft and begins to spread out as shown in Figure 12. Likewise the metal of the base wall 5 begins to soften and in a very short time the welding temperature in both parts is reached. Directly thereafter the current is turned ofi. After the weld is complete the metal of the element in many instances actually extends into the body of the base wall as indicated at H in Figure 7.

During the final stages of the operation, the chuck may seat firmly against the upset foot 8 to shape the same, as shown in Figure 13, it being understood that the .bottom of the chuck is appropriately formed.

All of the factors, namely-the current, the magnitude of the pressure or welding force, the length of the element protruding below the chuck and designated L, and the time for which the current is on, have an important bearing upon the success of the method. All of these factors vary with the cross sectional area of the element as will be seen from a consideration of the graphs shown in Figures -18. The curves of these graphs cover one example of the application of my invention, specifically copper elements on a steel base wall. In each instance the factor to be determined is plotted against the cross sectional area of the element, depicted as the abscissa of the graph and shown in mmfi. Thus for an element (copper) of mm. for instance, the optimum welding current would be 15,000 ainperes; the pressure or welding force, 200 the length L (projection of element from chuck towards base Wall) 4.8 mm.; and the welding time (current on) would be 0.25 sec.

The curves above referred to were derived through the use of the following formulae:

Welding time:

Welding current:

Frojecting length:

where A=cross sectional area of element in mm.

L=projecting length from the chuck in mm.

t=welding time in seconds I=welding current (the root mean square cur rent) in amps.

C1=electrical conductivity of element in percent of copper Cz=electrical conductivity of base wall in percent of copper F=welding force in kg.

and where the constants for C1 and C: are:

Copper=100 eluminum=65 Aluminum alloys= teel=15 Stainless steel Iii/8:2

As noted hereinbefore the area of the actual junction, which of course is the same as the area of the bottom of the foot or base, should not be less than one and three-fourths (1%) times the cross sectional area of the element per se.. It should also be observed that this area bears an inverse ratio to the cross sectional area of the element, i. e. for the small size elements the area of the base must be larger in proportion than for the large size elements. This follows from the fact that the smaller diameter wires have higher heat absorption than the larger "iameter wires so that there is more heat transferred to the base wall in the case of the smaller size elements. As pointed out in the aforesaid copending application the elements range in size between 3 mm. and 50 mm The 1.75 minimum ratio thus applies for the 50 mm. elements and for the smallest size (3 mm?) the ratio should be not less than about 2.5 for good heat transfer.

The thickness of the base wall is not too important. Ordinarily, of course, the thickness of the base wall will be determined by the pressures of the fluids being handled and obviously for best heat transfer conditions the we s should be as thin as possible as long as the deformed wall is not so thin that during the welding oper ation it will be collapsed or broken through.

While the example covered by the graphs and formulae given above specifically concern the application or" copper elements to a steel bas wall, the table of constants for Cl and includes aluminum and aluminum alloys for the elements, so that by substituting the appropriate constants for those of the given formulae, these formulae may be used for the attachment of aluminum elements to a steel base wall, and, of course, through the use of such revised formulae suitable curves can be drawn to correspond with those of Figures l5 l8.

From the foregoing description taken in connection with the accompanying drawings it vsil be readily apparent to those skilled in the art that this invention provides an exceptionally speedy method of securing extended surface elements to the base wall of a heat exchange device and similar composite article.

What I claim as my invention is:

l. The hereindescribed method 0 a composite article having a met 1 and a. plurality of closely $13868; ments of metal having an electrical conductivity of between percent and iii) percent that of copper projecting theircin, which method. coin- .rises: holding an element in a holder with one end portion or" the element p t a short distance from the pressing re projecting end t against the suriace oi the base wall by iii-cans of said holder; cussing an electric current of flow voltage and high amperage through the wall and the projecting end portion of the element while maintaining the pressure between the element and the base wall to thereby bring the contacting portions of the element and base wall to welding temperature soften the portion of the element lying heu n the base wall and the hoider; and mail aining said current and pressure until the portion of the element in juxtaposition to the base wall is spread out over and welded to the base wall.

2. The method defined in claim 1 characte ized by the fact that where the metal of the element has an electrical conductivity of between 100 and 30 percent of copper and the base wall is of metal having an electrical conductivity of between 1 and 20 percent of the conductivity of pure copper, the length of the element projecting from the chuck is determined by the formula the force with which the element is pressed against the base wall is determined by the formula the duration of time for which the current is on is determined by the formula and the current is a value determined by the formula where in each of the formulae A=cross sectional area of element in mm.

L=projecting length from the chuck in mm.

t=welding time in seconds I=welding current (the root means square current) in amps.

C1=electrical conductivity of element in per cent of copper Cz=electrical conductivity of base wall in per cent of copper F=welding force in kg.

3. The hereindescribed method of fabricating a composite article having a metal base wall and a plurality of closely spaced rod-like elements of metal having a coefficient of thermal conductivity projecting therefrom, which comprises: grasping an element in a chuck at a point close to one end of the element; cooling the chuck to preclude adhesion between it and the element grasped thereby and also to confine the annealing of the element resulting from the temperature rise therein to a short distance from the end of the element to be attached; bringing said end of the element against the surface of the base wall; developing a pressure between said element and the base wall sufficient for welding but insufiicient to deform either the base wall or the element; when said pressure is attained, passing an electric current of low voltage and high amperage through the Wall and the portion of the element between the chuck and the wall to thereby bring the contacting portions of the element and wall to welding temperature and soften the portion of the element lying between the base wall and the chuck; and maintaining said current and pressure until the portion of the element in juxtaposition to the base wall is spread out over and welded to the base wall.

4. The hereindescribed method of fabricating a heat exchange device which consists of a metal tube and a plurality of closely spaced rodlike extended surface elements of metal having a high coefficient of thermal conductivity attached thereto and projecting therefrom with the points of attachment of the elements substantially equally distributed around the circumference of the tube, which method comprises: grasping each element individually in a chuck at a point close to one end of the element; pressing said end of the element against the surface of the tube; while the element is pressed against the tube passing an electric current of low voltage and high amperage through the tube wall and the portion of the element between the chuck and the wall to effect softening of said intervening portion of the element and to bring the contacting portions of the element and tube wall to welding temperature; maintaining the current flow and the pressure on said element until the portion of the element in juxtaposition to the base wall is spread out over and welded to the base wall and the adjacent portion of the element is annealed; cooling said chuck to preclude adhesion between it and the element grasped thereby and also to confine the an nealing of the element resulting from the temperature rise therein to a short distance from the end of the element being attached; and bending the attached elements at their annealed portions to dispose all of the elements in parallel relationship.

5. The hereindescribed method of fabricating a composite article having a metal base wall and a plurality of closely spaced rod-like elements of metal having a high coefficient of thermal conductivity projecting therefrom, which comprises: holding an element in a holder with one end of the element exposed; bringing said end of the element against the surface of the base Wall; imposing an endwise compressive force upon the element to develop a pressure between it and the base wall suificient for welding but insufficient to deform either the wall or the element; while said pressure is maintained passing an electric current of low voltage and high amperage across the junction between the element and the base wall to thereby bring the contiguous portions of the element and wall to welding temperature; and maintaining said current and pressure until that portion of the surface of the base Wall against which the element is pressed and that portion of the element pressed thereagainst becomes plastic and forms an enlarged head coaxial with the element, and welds said head to the base wall with a portion of the head projecting below the surface of the base Wall.

6. The hereindescribed method of making a heat exchanger unit having a metal base wall and rod-like extended surface elements of metal having an electrical conductivity of between 100 per cent and 30 percent that of copper, projecting from the base wall, which method comprises: pressing the rod-like extended surface elements endwise against the surface of the base wall with a force determined by the formula where A=cross sectional area of element in mm, and F=we1ding force or pressure in kg.;

and when said pressure is attained and while it is maintained, passing an electric current of low voltage and high amperage across the junction between the element and the base wall to thereby bring the contiguous portions of the element and base wall to welding temperature and soften the same; and maintaining said pressure and current until the portion of the element contiguous to the base Wall is spread out over and welded to the base wall.

7 The hereindescribed method of making a heat exchanger unit having a metal base wall and rod-like extended surface elements of metal having an electrical conductivity of between 100 percent and 30 percent that of copper, projecting from the base wall, which method comprises: pressing the rod-like extended surface elements endwise against the surface of the base wall with a force sufiicient for welding but insufficient to deform either the base wall or the element; and when said pressure is attained and while it is maintained, passing an electric current of avalue determined by the formula where I=welding current (the root mean square current) in amps. t welding time in seconds Ci electrical conductivity of the element in per cent that of copper Cz electrical conductivity of the base wall in per cent that of copper A=cross sectional area of the element in mm.

across the junction between the element and base wall to thereby bring the contiguous portions of the element and base wall to welding temperature and soften the same; and maintaining said pressure and current until the portion of the element contiguous to the base wall is spread out over and welded to the base wall.

8. ihe hereindescribed method of making a heat exchanger unit having a metal base wall and rod-like extended surface elements of metal having an electrical conductivity of between 100 percent and 30 percent that of copper, projecting from the base wall, which method comprises:

pressing the rod-like extended surface elements t=0.05i0.05+ (13:7) XAX- where t welding time in seconds, and A cross sectional area of element in mm 9. The hereindescribed method of making a heat exchanger unit having a metal base wall nd rod-dike extended surface elements of metal aving an electrical conductivity of between 160 crcent and percent that of copper, proiect- 111g from the base wall, which method comprises:

pressing the rod-like extended surface elements endwise against the surface of the base wall with a force determined by the formula F: (7i) (lO-i-A) and when said pressure is attained and while it is maintained, passing an electric current of a value determined by the formula I= [1occ+ 7oi5o+4.1 A]

W 2 across the junction between the element and base wall for a period of time determined by the formula where in each of said formulae lfl=cross sectional area of the element in mm.

t=welding time in seconds I=welding current (the root mean square current) in amps.

Ci=electrical conductivity of the element in per cent that of copper oz=eiectr1cal conductivity of the base wall in P cent of copper F=welding force or pressure in kg.

e amperage across the junction I2 10. The hereindescribed method of securing a rod-like element of copper or similar high conductivity metal to a steel wall with the element projecting substantially perpendicularly from the wall, which comprises: pressing the element against the surface of the wall with a pressure determined by the formula F=(7- *-3) (10+A) and when said pressure is attained passing an electric current of a value determined by the formula across the junction between the element and the base wall, for a period of time determined by the formula t=0.05i0.05+(131:7) A 10- where in each of said formulae A==cross sectional area of the element in mm.

t welding time in seconds I=welding current (the root mean square current) in amps.

C1=electrical conductivity of the element in per cent that of copper Cz=electrical conductivity of the base wall in per cent that of copper F=welding force or pressure in kg.

11. The hereindescribed method of making a boiler tube having rod-like extended surface elements of high thermal conductivity projecting from its external surface, which comprises: pressing a. rod-like extended surface element against the surface of the tube with a pressure determined by the formula when said pressure is attained and while it is maintained, passing an electric current of a value determined by the formula across the junction between the element and the tube, for a period of time determined by the formula where in each of said formulae A=cross sectional area of the element in mm.

t=welding time in seconds I=welding current (the root mean square current) in amps.

C1=e1ectrical conductivity of the element in per cent that of copper Cz,,=clectrical conductivity of, the base wall in per centthat of copper F=welding force or pressure in kg.

and repeating said procedure with each element until the required number of elements has been applied to the tube,

12. The hereindescribed method of fabricating an extended surface heat exchanger including a metal base wall and a plurality of closely spaced metallic rod-like elements projecting therefrom, which comprises: engaging a metallic rod-like element in a. holder in a manner providing good electrical contact between the holder and the element and with one end of the element projecting from the holder a. short distance; bringing the protruding end portion of the element against the surface of the base. wall; developing suflicient pressure between said protruding end of the element and the base wall to provide a good electrical contact therebetween but without deforming either the element or the base wall; passing an electric current of low voltage and high amperage through the base wall and said protruding portion of the element lying between the holder and the wall when said pressure has been attained to thereby bring the contacting portions of the element and base wall to Welding temperature and soften the end portion of the element adjacent to the base wall; cooling the holder to preclude adhesion between it and the element held thereby and further to confine the annealing of the element resulting from the temperature rise therein to the said end portion of the element projection beyond the holder; and maintaining said pressure and current until substantially the entire end face of the element press- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 455,420 Thomson July 7, 1891 1,062,571 Murray et al. May 20, 1913 1,887,513 Nelson Nov. 15, 1932 2,003,320 Trainer et al. June 4, 1935 2,220,579 Murray Nov. 5, 1940 2,231,480 Pilger Feb. 11, 1941 2,327,924 Mounts Aug. 24, 1943 2,337,294 Cooper Dec. 21, 1943 

